Space based augmentation systems or SBASs are satellite radionavigation systems intended to supplement systems ensuring a basic satellite navigation service or GNSS for “Global Navigation Satellite Systems” such as the GPS, GALILEO, or GLONASS systems so as jointly to provide superior performance in terms of location precision, availability and continuity of service and integrity of the information provided.
These systems transmit on one or more satellites (typically geostationary) an L-band signal transporting in particular a string of navigation messages or NOF for “Navigation Overlay Frame”, at a rate of one message per second.
This signal and the string of messages transmitted are defined by an international standards document RTCA MOPS DO-229-D, entitled “Minimum Operational Performance Standards for Global Positioning System/Wide Area Augmentation Equipment”.
The known basic architecture of such a space based augmentation system is described in the European patent application published under the number EP 2 579 062 A1.
According to this architecture, a signal, transmitted by satellites of a constellation of a GNSS satellite navigation system, is received by a set of reception and observation ground stations, or RIMSs for “Ranging and Integrity Monitoring Stations”, dispersed over a wide territory (for example Europe). These RIMS stations transmit signals corresponding to the signals received, through a long-distance network or WAN for “Wide Area Network”, to a computation centre or CPF for “Central Processing Facility”. This computation centre formulates corrections and integrity data making it possible to provide the user with the required performance of the navigation service and transmits at each computation cycle lasting a duration of a second a part thereof in the form of digital navigation messages or NOF for “Navigation Overlay Frame” which are transmitted through the long-distance network to a broadcasting ground station or NLES for “Navigation Land Earth Station”. This broadcasting ground station transmits signals corresponding to the signals received, to geostationary satellites which rebroadcast them via broadcasting signals to receivers of users of the service. The receivers of the users simultaneously receive the first signals of the geostationary SBAS satellites, generally geostationary, and the second signals of the satellites of the constellation of the GNSS satellite navigation system, and each compute their position with the aid of these two types of signals. It should be noted that the RIMS reception and observation ground stations also receive these signals and that they transmit the digital navigation messages NOF thereof to the computation centre CPF jointly with the information received from the satellites of the GNSS constellation.
The computation cycle described hereinabove is performed by the space based augmentation system SBAS in a repetitive manner typically every second.
The implementation is often pipelined, each element performing during a computation cycle the processing of the data which will be processed by the following element during the following cycle.
It should be noted that at the user level the knowledge of a string of streams of navigation messages NOF of a certain length (typically several minutes) is necessary to compute the positioning thereof. The coherence of the various streams of successively transmitted navigation messages NOF is therefore a major issue: this is why one speaks of a string of transmitted navigation messages NOF, and not of isolated navigation messages NOF.
A basic implementation such as this does not make it possible to provide the very short-term availability and the continuity that are expected by the users of such a system: typically an availability of the order of 99% and a probability of loss of continuity of better than 10−5/h are characteristic of the expected performance for the currently most widespread use, namely that of civil aeronautics.
In particular a fault with the main computation centre, that is to say with the computation centre intended to operate nominally in master mode, gives rise to the interruption of the transmissions of the stream of navigation messages or NOF and an immediate loss of continuity with an impact on availability.
It is known to use SBAS space based augmentation systems with redundancy, the EGNOS system being an example of one such redundant system 2, represented here in a simplified manner in FIG. 1 with only two computation centres 12, 14, geographically remote from one another, instead of the three computation centres used in practice.
According to FIG. 1, the computation centres 12, 14, installed respectively on a first site called “site 1” and on a second site called “site 2”, receive from the set 20 of RIMS stations and respectively in parallel the navigation data signals of an integer number N, here at least equal to three, of geostationary satellites 22, denoted NOF 1, NOF 2, . . . , NOF N, and the navigation signals of the satellites 24 of the GNSS constellation.
As a function of these received signals, the first and second computation centres 12, 14 independently formulate their streams of respective navigation messages, NOF 1.1, NOF 1.2, . . . , NOF 1.N for the first computation centre 12, NOF 2.1, NOF 2.2, . . . , NOD 2.N for the second computation centre 14, which are each transmitted to a broadcasting ground station NLES#j, that is to say the broadcasting station having the same index j as the signals NOF 1.j, NOF 2.j. For example, the first computation centre 12 transmits the stream of messages NOF 1.2 to the broadcasting station of rank 2 while the second computation centre 14 transmits the stream of messages NOF 2.N to the station of rank N.
Each broadcasting ground station NLES#j, j varying from 1 to N, selects one of the streams of navigation messages from among the streams of messages NOF1.j, NOF 2.j that it receives and transmits it to the receivers of users, by way of a geostationary satellite of the same index j with which the broadcasting ground station NLES#j is associated through the plan of the transmission resources of the said satellite.
Each broadcasting ground station NLES#j also receives, continuously from each computation centre 12, 14, integrity data representative of the integrity of the streams of messages NOF 1.j, NOF 2.j, received by the station NLESj and dispatched by the first and second computation centres 12, 14, these integrity data being denoted respectively by DI 1.j for the stream of messages NOF 1.j transmitted by the first computation centre 12 and by DI 2.j for the stream of messages NOF 2.j transmitted by the second computation centre 14.
Each computation centre 12, 14 comprises respectively a unit 52, 54 for processing and formulating the streams of navigation messages, the streams of messages NOF 1.j being formulated by the unit 52 of the first computation centre 12 or CPF#1, and the streams of messages NOF 2.j being formulated by the unit 54 of the second computation centre 14 or CPF#2.
Each computation centre 12, 14 comprises respectively an integrity data formulation unit 62, 64 for the formulation of the integrity data DI 1.j, DI 2.j. 
In case of fault with one of the computation centres 12, 14, each broadcasting ground station NLES#j selects the message of one of the other computation centres or CPFs in a good operating state, thus maintaining continuity of transmission of navigation message streams or NOF destined for the receivers of users.
The computation centres 12, 14 are situated on different geographical sites which are sufficiently far apart to avoid a fault that would cause complete breakdown in case of a local major problem (fault with the communication network over a country, major industrial accident, natural catastrophe, . . . ).
The corresponding conventional architecture of the SBAS system and its corresponding method of implementation such as are described hereinabove and in FIG. 1 use an approach based on an independence between the streams of messages broadcast by the space based broadcasting means, namely the satellites 22.
Thus, in general a navigation message, which is substantially different especially in terms of sequence of message types, is generated by geostationary satellite, thereby giving rise within one and the same computation centre to a proliferation of independent processing pathways or channels, associated with the streams of messages received by the RIMS reception stations 20, as a function of the number of geostationary satellites, and also an increase in the exchanges of signals between the various functional entities of the EGNOS system for the analysis of possible faults.
It is thus sought to decrease the volume of the analyses to be performed on the observation data received by the RIMS reception and observation stations when the number of geostationary satellites increases, this decrease having already to be effective when the number of satellites goes from one to two.
The technical problem is to decrease the complexity of the architecture of an SBAS system, in particular the complexity of the architecture of its computation centres or CPFs when the number of its broadcasting means, for example geostationary satellites, increases without impairing the usual performance in terms of precision, reliability, continuity and availability, conventionally demanded of an SBAS system.
Correspondingly, the technical problem is to decrease the complexity of the method for analyzing the GNSS navigation data and the SBAS navigation data and of the method for generating the streams of SBAS navigation messages, implemented by the SBAS system when the number of its broadcasting means increases, without impairing the usual performance in terms of precision, reliability, continuity and availability, conventionally demanded of an SBAS system.